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Building Telephony Systems with OpenSER by Flavio E. Goncalves

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SIP Transactions and Dialogs

SIP Transactions and Dialogs

It is important to understand now the difference between a transaction and a dialog. A transaction occurs between a user agent client and a user agent server and comprises all messages from the first request to the final response. The responses can be provisional starting with 1 followed by two digits (e.g. 180 Ringing) or final starting with 2 followed by two digits (e.g. 200 OK). The scope of a transaction is defined by the stack of VIA headers of the SIP messages. So, the user agents, after the initial invite, don't need to rely on DNS or location tables to route the messages.

A dialog usually starts with an INVITE ...

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